Spaced anode assembly for diaphragm cells

ABSTRACT

Anolyte circulation in electrolytic cells equipped with anode blades which expose more than 18 inches of active surface height is improved by providing a circulation space between each anode blade in a bank of anodes. The distance between anode blades varies with the type of cathode construction, to provide between 3/4 inch to 3 inches distance in cells employing cathodes which provide for a center circulation aisle and between 1/4 inch to 3 inches distance in cells employing cathodes which extend across the cell.

United States Patent [72] Inventors John E. Currey 2,987,463 6/l96l Baker et al 204/266 3,485,730 12/1969 Virgil 204/l28 OTHER REFERENCES Stender et al., Reprint, The Electrochemical Society Journal," I934,pp. 1-24 Primary Examiner-T. Tung Alrorneys- Peter F. Casella, Donald E. Studley, Richard P.

Mueller, James Mudd and Richard K. Jackson Lewiston; Walter W. Ruthel, Niagara Falls, both of N.Y. [2i] Appl. No. 733,988 [22] Filed June 3, 1968 [45] Patented Nov. 2, 1971 [73] Assignee Hooker Chemical Corporation Niagara Falls, N.Y.

[54] SPACED ANODE ASSEMBLY FOR DIAPHRAGM CELLS 4 Claims, 3 Drawing Figs.

[52] [1.5. CI. 204/252, 204/98, 204/258, 204/266, 204/278 [51] int.Cl com 1/06 [50] Field oi Search 204/98-100, I28, I29, 252, 256, 258, 266, 270,

[56] References Cited UNITED STATES PATENTS 2,370,087 2/1945 Stuart 204/266 PATENTEU NUV2 I971 SHEET 2 BF 2 SPACED ANODE ASSEMBLY FOR DIAPHRAGM CELLS This invention relates to improved electrolytic cells for the electrolysis of aqueous electrolyte-containing solutions and more particularly to the anode assembly of a diaphragm-type electrolytic cell particularly suited for the electrolysis of alkali-metal chloride solutions.

Chlor-alkali diaphragm cells have been used extensively for many years for the production of chlorine, caustic and hydrogen. Over the years, such cells have been perfected to a degree whereby extremely high operating efficiencies are obtained, based upon the electrical energy expended. Most recent developments in diaphragm-type chlor-alkali cells have been improvements which increase the production capacity of an individual cell, resulting in an increased production rate or capacity for a given cell room area. Recently developed chloralkali cells are capable of utilizing over 55,000 amperes of current per cell. Various structural improvements are needed in chlor-alkali cells using over 55,000 amperes of current per cell in order to maintain or increase their current and power efficiencies when compared to more conventional lower amperage cells such as those utilizing about 30,000 amperes. The mere enlargement of the component parts of such cells, while providing highly efficient cells, does not always provide the most favorable efficiencies, based upon construction costs and operating perfonnance.

A problem in the development of highly efiicient electrolytic cells resides in providing for adequate circulation of electrolyte. Tl'le circulation of electrolyte is directly related to cell efficiency. The Hooker type-S cell which operates conventionally on about 30,000 amperes of current per cell provides space for electrolyte circulation through a center circulation aisle. The electrolyte circulates laterally between the electrode and the center circulation aisle. A discussion of the Hooker type-S cell may be found in Sconce, Chlorine, It's Manufacture, Properties and Uses, A.C.S. Monograph 154, pp. 97 i962) Reinhold Publishing Corporation.

The anode assembly in the conventional chlor-alkali cell equipped with a center circulation aisle, is formed with abutted graphite anode blades, machine notched at their base to insure adequate mastic coverage of the graphite lead bond. The anode blades are arranged so that they appear in a series with their edges touching in such manner as to provide a center circulation aisle in combination with the cathode fingers. Chlorine gas forming at the face of the anode blades rises, creating a gas lift action vertically directed along the faces of a bank of anodes. This gas lift action draws fresh anolyte laterally along the lower portion of the anode blades from the center circulation aisle (downcomer space). The fresh electrolyte then flows vertically along the anode face into the region above the anodes at which point chlorine gas is leaving the electrolyte. The heavier electrolyte, from which chlorine gas has been partially exhausted flows laterally, above the anode bank to the downcomer center circulation aisle.

It has been discovered that, in electrolytic cells containing vertically disposed anode blades which extend into the electrolyte to provide an active surface greater than about 18 inches in height, the lateral flow of electrolyte toward the center circulation aisle near the top of the electrodes and from the center circulation aisle near the bottom of the electrodes is greatly diminished, resulting in decreased overall cell efficiency. in fact, anolyte circulation in an electrolytic cell equipped with a bank of anode blades exposing more than about 18 inches of active electrode height occurs primarily by vertical cascading movements of electrolyte over the electrode faces rather than by lateral circulation to and from the center circulation aisle. When anode blades are employed which approach 27 inches or more of exposed surface height, anolyte circulation diminishes to a small cascade at the top of the electrode blade near the center circulation aisle. This anolyte circulation problem is more severe in electrolytic cells provided with no center circulation aisle. The problem of electrolyte circulation is a result of the size of an anode blade rather than the material from which the anode is constructed. Although, graphite is referred to throughout this disclosure, it is to be understood that the anode may be of any commonly employed material such as magnetite, graphite and the platinum or palladium metals or their oxides employed with supporting substrate metals such as titanium, tantalum and the like.

In accordance with this invention the anode system for use in electrolytic cells in which the active surface of the vertical anode blade is greater than about 18 inches in height, has been designed to provide circulation space between adjacent anode blades rather than in a center circulation aisle. ln providing this spaced anode system it has been determined that a minimum space of about one-quarter inch should be used to obtain good mastic coverage of the lead base supporting the vertical blade anodes. The maximum width of the spacing of the anode blades is optional commensurate with considerations of dead screen area of the cathode and the expense of greater cathode area. With an excessive amount of dead screen area on the cathode, more unreacted anolyte will tend to filter across the diaphragm and dilute the cell liquor.

Because of the overvoltage characteristics of all commonly used electrodes, the higher the current density on the anode in relationship to the current density of the cathode, the greater the ratio of chlorine to oxygen discharged at the anode. In other words, the greater the current density on the anode, the greater the current efficiency of the cell. Since an anode has a certain amount of current-throwing capability, the cathode screen area not directly facing an anode in the spaced anode system of the instant invention, does conduct current. However, the current density of the anode is increased in relation to the cathode in a spaced anode assembly and consequently the cell with spaced anodes will give a higher current efficiency, less graphite wear and improved electrolyte circulation when compared to a cell equipped with unspaced anodes.

More specifically, this invention provides an anode system, for electrolytic cells equipped with anode blades presenting more than about 18 inches of active electrode height and a cathode structure that extends across the cell without provision for a center circulation aisle, of spaced vertical anode blades. The space between adjacent anode blades may be from one-quarter inch to 3 inches with anodes which range in width from 4 to 10 inches. Preferably the space between adjacent anode blades is between one-half inch to 2 Inches when commercially common sizes of graphite from 6 to 8% inches in width are employed.

Likewise, in electrolytic cells equipped with cathode fingers which do not extend across the cell, but form a center circulation aisle, this invention provides optimum anolyte circulation via an anode system in which the spacing between adjacent anode blades is between three-fourth inch to 3 inches. Preferably the spacing of anode blades in cells equipped with a center circulation aisle is between three-fourth inch to 2 inches.

Advantages resulting from a spaced anode system for electrolytic diaphragm cells with anode blades or more than about 18 inches in height reside in the increased anolyte circulation providing a more uniform anolyte composition, more uniform wear of the anode and higher cell efficiency. With a more uniform anolyte composition in the cell, the current efi'iciency may be increased by about 0.5 percent, or from to 95.5 percent. A 0.5 percent savings in current represents a significant cost savings. Likewise, the inefficiency of the electrolysis is reduced from about 5 percent to about 4 percent, or a reduction of about 10 percent. This reduced inefficiency is approximately proportionate to graphite consumption and a l0 percent savings in graphite consumption is a significant attribute of the improved anolyte circulation resulting from the instant invention. Furthermore, undercutting of the anode blade, requiring premature removal and replacement of the anode, is diminished significantly. Initially, a smaller graphite inventory is needed for each cell when compared to the conventionally used unspaced anode bank. For example. a 2 -inch spacing with anode blades 8 inches wide represents an initial 20 percent savings in graphite with the same amount of chlorine production, and an increased current efficiency.

Another advantage resulting from the spaced anode system of this invention is a reduction in the cost of each anode blade by elimination of the previously. essential blade notching, which insured an adequate lead and mastic surrounding for anode blade systems where the blades are placed in a row with their edges in contact. The savings resulting from blade spacing and elimination of notching alone, amounts to from 1 to 2 cents per pound based upon a conventional graphite anode blade.

Another advantageresulting from a spaced anode system in conjunction with a cathode screen extending across the cell without provision for a center circulation aisle resides in the reduction of cell width and consequently the cost of a cell. Actually, since electrolyte circulation is uniform throughout the cell that embodies the spaced anodes of this invention, the only limitation upon the width of a cell and an anode bank may be relegated to the current-conducting capacity and structural strength of the cathode.

DETAILED'DESCRIPTION OF THE INVENTION This invention may best be understood by reference to the accompanying drawings in which:

FIG. 1 represents a partial plan view of an electrolytic cell with a center circulation aisle, showing the cathodic and anodic electrodes and then spacing with respect to each other.

FIG. 2 is an enlarged section of FIG. 1 taken along line 2- FIG. 3 represents a partial plan view of the anode assembly in an electrolytic cell having cathodes extending across the cell.

The instant invention is applicable to many different electrolytic processes. It is to be understood that the description of a diaphragm-type chlor-alkali cell is not a l limitation on the invention because the anode system is applicable in the operation of electrolytic cells which do not employ a diaphragm in the electrolysis of substances other than alkali-metal chlorides in chlor-alkali preparation.

With reference to FIGS. 1 and 2, the cathode shell contains the cathodes 12 which extend into the cell to form a center circulation aisle 14 as depicted in FIG. 1. The anode blades 16 oppose the cathode finger faces. Each anode blade is spaced 18 at its side from an adjacent anode blade, to provide for anolyte circulation. This spacing is from about threequarter inch to 3 inches in the cell depicted in FIGS. 1 and 2 wherein a center circulation aisle is present. With specific reference to F IG. 2, the spaced anode blades 16 are vertically disposed from the cell bottom 20 in which they are fixed in a lead base 22. The anode-lead juncture is sealed from the electrolyte by an asphalt or mastic bituminous material 24. Other materials known to the art may be used for this sealing-insulating purpose. The electrolyte covers the anode blades and extends upward into the region of the cell top 26. The circulation of anolyte over the spaced anodes is depicted by way of arrows to indicate the downward flow of electrolyte in the spaces between anode blades and the upward sweep of electrolyte over the electrode face. This flow of electrolyte upward tends to sweep chlorine from the electrode face and the lighter chlorine-containing liquid rises while the heavier electrolyte containing less gas circulates downward through the spaces between the electrodes.

The cathode she'll 28 presented in FIG. 3, houses the cathode screen 30 which extends across the cell without provision for a center circulation aisle. The anode blades 32 are spaced 34 to provide for the improved electrolyte circulation and overall increased cell efficiency of this invention. Although, banks of eight anode blades are depicted in FIG. 3, it is to be understood that any reasonable number of anode blades may be aligned in a bank with spacing between the anodes of from about one-quarter inch to 3 inches where a centercirculation aisle has been omitted.

Having described the invention, it Wlll become obvious to those skilled in the art that modifications of the invention may be made which do not depart in spirit from the true scope of this invention.

We claim:

1. An electrolytic cell comprising a container, a cathode assembly, said cathode assembly comprising a plurality of cathodic structures of internally reinforced foraminous screen fingers which extend into the interior of said container, across the cell, and an anode assembly, said anode assembly comprising a plurality of vertically disposed anode banks positioned opposite the cathode screen fingers, each bank being substantially parallel to an adjacent cathode finger and each bank being comprised of a plurality of anode blades, each of which presents an active surface height of at least about 18 inches, each anode blade in each bank being spaced in such a manner that the distance between an edge of an anode blade to an adjacent edge of an adjacent anode blade in the same bank is from three-quarter inch to 3 inches in a cell of center circulation aisle fingered cathode construction and from one-quarter inch to 3 inches in a cell of continuous fingered cathode construction to form a passage between a pair of adjacent anode blades through which anolyte may be circulated, each anode bank having at least two of said passages and which passages have substantially the same width.

2. The electrolytic cell as claimed in claim 1 in which the cell is a diaphragm cell and each anode blade presents an active surface height of about 27 inches.

3. The electrolytic cell as claimed in claim 2 in which the cell is of the center circulation aisle fingered cathode construction and in which the width of the passage between two adjacent anode blades is from about 1 to 2 inches.

4. The electrolytic cell as claimed in claim 3 in which the cell is of the continuous fingered cathode construction and the width of the passage between two adjacent anode blades is from about three-quarter inch to 2 inches.

# I i O i 

2. The electrolytic cell as claimed in claim 1 in which the cell is a diaphragm cell and each anode blade presents an active surface height of about 27 inches.
 3. The electrolytic cell as claimed in claim 2 in which the cell is of the center circulation aisle fingered cathode construction and in which the width of the passage between two adjacent anode blades is from about 1 to 2 incHes.
 4. The electrolytic cell as claimed in claim 3 in which the cell is of the continuous fingered cathode construction and the width of the passage between two adjacent anode blades is from about three-quarter inch to 2 inches. 